The NHS has spent £4million on sending fat people to Weight Watchers over the past five years.GPs now routinely refer patients to the classes, which cost 'private' visitors around £45 for three months.

Recent studies have found the courses to be highly effective at helping people lose weight.

GPs have been able to send patients to them on the NHS since 2007, after they were approved by the rationing watchdog NICE. They are offered by two-thirds of Primary Care Trusts.

Obese: The NHS has spent £4million on sending fat people to Weight Watchers over the past five years

Now figures obtained by Channel 4 documentary programme Dispatches show that the NHS spent £4million on Weight Watchers between 2007 and 2012 – the equivalent of £800,000 a year.

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It is estimated that 30,000 are sent on courses paid for by the Health Service each year, as it attempts to overcome the country's obesity crisis.

Celebrity slimmer: Patsy Kensit was unveiled as one of Weight Watchers' newest ambassadors, having lost 10lb in 12 weeks following the points system

Nearly a quarter of adults are classified as obese – having a body mass index above 30 – and experts predict this will increase to a half by 2040.

Recent research has suggested that Weight Watchers classes are more effective for weight loss than following dietary advice from a doctor.

In addition, in December a study conducted by the NHS found that patients lost an average of three pounds more on Weight Watchers than they did after visiting other so-called slimming clubs.

The analysis showed that Weight Watchers members were 81 per cent more likely to shed 5 per cent of their weight – an amount judged by experts to make an impact on health – than those who went to rival company Slimming World.

The study's authors said the secret of Weight Watchers success may be that people seemed to find its course easier to complete than those of its competitors.

Similar work by the Medical Research Council in 2010 found that patients on a Weight Watchers course lost twice as much weight as those who merely sought advice from their GP. Researchers have also said courses give patients good habits for life.

However, concerns have been raised that many pro-Weight Watchers studies are funded by the company itself, and may therefore be biased.

Claire Friedemann, of the centre of evidence-based medicine at the University of Oxford, told Dispatches: 'The danger with companies funding their own research is that they may only publish results which are positive for them.'

After analysing ten studies that suggested Weight Watchers was effective, she found that eight had been paid for by the firm itself.

Her analysis also showed that while many Weight Watchers patients lost weight within the first three months, some had put it back on again after five years.

Battling the bulge: Nearly a quarter of adults are obese and experts predict this will increase to a half by 2040

In addition, she warned that some Weight Watchers data may be inaccurate because it is based on patients' own measurements.

A spokesman for the company said: 'Weight Watchers has a transparent and credible approach to science.

'We work with world-renowned obesity researchers who run independent studies.

'Evidence from published papers demonstrates that the majority of those who lose weight with Weight Watchers' help do not regain it after the programme ends.'